Basic working principle of surface plasmon resonance (SPR) sensor is inducing a spectrum of light into a sensing region and analyzing the absorption spectrum from the sensing region. Generally, inducing a spectrum of light on a sensing layer can be achieved in two ways; shining single wavelength of light with a range of incident angle or inducing a wavelength range of light. Sensor type is divided into two correspondingly; reflection type and waveguide type.
The reflection type SPR sensor 10, as shown in FIG. 1, works by applying single wavelength of light on the back side of a sensing layer with a range of angle and measuring the shifted amount of minimum light reflection angle before and after an analyte is induced to detection layer. The minimum light reflection angle is related with analyte's refractive index and detection layer metal. If the refractive index of analyte changes, the refractive index change would be reflected on the shift of minimum reflection angle. A conventional and commercially available SPR sensor 10 consists of optical components including a light source that produces a polarized light 12, a prism 14, and a CCD array 18. The metal film 22 is prepared to have a functionalized surface for the adsorption of biochemical molecules from a fluidic sample 24. The light source generates a polarized light 12 which is directed through the prism 14, striking the metal film 22. Reflected light 16 is detected by the CCD array 18. As the fluid sample 24 passes through the fluidic channel 26, the binding of the molecules changes the refractive index, which is monitored, conventionally, by the shift of the minimum reflection angle.
An advantage of the reflector SPR sensor is that it may include plural interactive surfaces, metal films 22, to allow a multiple channel analysis of the sample 24. However, a disadvantage of the reflector sensor is that the sensor is not fully integrated on a planar surface. Instead, optical components are located a distance from the planar surface to provide a polarized light that strikes, and is reflected from, the metal film 22. Since the reflector type sensor is based on the measurement of the reflected light intensity, the CCD array for monitoring minimum light reflection peak is also located outside the plane. Thus, the reflector SPR sensor is “bulky”.
Miniaturized reflector type SPR sensors are disclosed in U.S. Pat. Nos. 6,183,696 issued to Elkind et al on Feb. 6, 2001 and 6,191,847 issued to Melendez et al. on Feb. 20, 2001. The miniaturized sensors include a substrate which provides a sensor platform to which a light transmissive housing is coupled, substantially encapsulating the sensor platform. A light source is provided above the platform or on the platform substrate and includes a polarizer for producing the polarized light that strikes an SPR layer which is formed on the exterior surface of the housing. A mirror, also located on the interior surface of the housing, deflects the light reflected from the SPR layer to a detector located on the sensor platform. The miniaturized reflector SPR sensor disclosed in the prior art may also include a power source, conversion electronics and a communication interface on the platform. Although most of the components are located on the platform, the housing is still necessary for contacting the target sample and reflecting the polarized light to the detector. While the overall size of the reflective SPR sensor is reduced, the resulting device is still bulky.
Another type of SPR sensor is waveguide type, which includes optical fiber type waveguide and planar waveguide. Fiber optic waveguides have a number of advantages over the bulkier prism-based sensors. Primarily, they can perform long distance detection for medical or otherwise sterile tasks. Fibers are also very small and have no moving parts, giving them a much broader range and making multiple sensor arrays a possibility.
A surface plasmon resonance sensor including plural optical waveguides and corresponding SPR sensor areas that permits the excitation of surface plasmons and plural fluidic channels is disclosed in U.S. Pat. No. 6,373,577 issued to Bräuer et al. on Apr. 26, 2002. In the Bräuer patent, an array of SPR waveguides are manufactured using technologies from semiconductor production and from integrated optics to provide plural parallel sensors on a single substrate located at a predefined distance from one another. Each strip-like optical waveguide that is expected to contact the sample fluid has at least one SPR sensor area including a metal layer that permits the excitation of surface plasmons. While the optical sensor disclosed in Bräuer provides plural waveguides with SPR surface areas and plural fluidic channels, external out-of-plane components are required to use the Bräuer optical sensor, namely, a light source, photodetectors and electronic components associated with operating the measurement device to which they are interfaced.
An alternative optical SPR detection device that is based on semiconductor laser array is disclosed in U.S. Pat. No. 6,469,785 issued to Duveneck et al. on Oct. 22, 2002. The Duveneck device comprises at least one light source, photodetector, an optical waveguide with a corresponding SPR area, and a fluidic channel in a single housing. The light source is a surface-emitting semiconductor laser located on a bottom substrate with the photodetector. An intermediate substrate above and separate from the bottom substrate includes at least one coupling-in grating and one coupling-out grating for coupling-in the emitted light from the surface-emitting laser to the optical waveguide and coupling-out to the photodetector. The optical waveguide includes a SPR surface area which contacts the target sample. A third substrate may be included a distance above the second substrate to form a fluidic reservoir for holding the target sample while in contact with the SPR sensor area.
While the Duveneck device may be enclosed to form a single unit, the device is bulky since the device requires a first substrate for the light source and detector and a separate second substrate for the coupling-in and coupling-out gratings, optical waveguide and corresponding SPR surface area. In an embodiment, the first, second and optional third substrate are enclosed in a housing with each substrate separated a predefined distance. In a second embodiment, the second substrate with the sensor layer is removable from the light source and detector on the first substrate. In still another embodiment, optical waveguides, such as an optical cable, are used to pass the light from the light source on the first substrate to the waveguide on the second substrate and back to the photodetector, thereby controlling the beam of the light. In the later embodiment, the first substrate may be located a further distance from the second substrate.